Such fluid compressors are generally termed turbocompressors or centrifugal compressors. They are equipped, with a stator and a rotor forming a permanent-magnet synchronous motor (brushless motor). Compressors of this type can reach very high speeds, for example 100,000 to 500,000 rpm. The motor drives the high-speed turbine, the turbine compressing the fluid. The fluid can be air, water, a gas, a refrigerant or any other suitable fluid. These compressors are used in numerous industrial, medical, pharmaceutical, foodstuff, automobile applications, in particular for supplying compressed air, or in refrigeration, heating or air-conditioning applications, for supplying compressed fluid.
In these compressors, the motor shaft is mounted rotatably on a frame by means of two axial bearings. These bearings can comprise ball bearings. However, it is difficult to obtain speeds of rotation with such ball bearings because of the difference in speed between the balls and the ball races. Furthermore, although the balls used are made of ceramic, the life span of such bearings is limited to about one hundred hours because of the high speeds of rotation. Other types of bearing can be used, such as aerodynamic bearings. However, this type of bearings has the disadvantage of being displaced transversely at the moment of start-up or during a change in speed, which creates friction at the level of the bearing elements.
Furthermore, compressors are generally lubricated by means of a lubricant. The disadvantage is that the lubricant risks becoming mixed with the fluid such that the compressed fluid is polluted by the lubricant. This is particularly dangerous, for example in the case of medical applications, such as dental applications, for which the compressed air reaching the mouth must be safe.
Finally, the compressors are generally used in installations with very large dimensions, the compressor being at a remove from the equipment which requires compressed fluid. The compressed fluid is supplied by means of a supply circuit provided in the network. Generally, this supply circuit is long, which involves risks of leakages of fluid along the circuit. Leakages in a compressed air circuit involve losses of pressure, which generates very high financial losses. Furthermore, because of the distance separating the compressor from the equipment which requires the compressed fluid, the network is permanently maintained under pressure in order to be able to respond rapidly as needed to the equipment. The permanent functioning of the compressor represents a high consumption of electrical energy.